The present invention describes a process for conditioning organic pigments in microreactors.
Organic pigments are well known and widely used for pigmenting macro-molecular organic materials such as paints, plastics or printing inks. Assynthesized, they are usually either very fine prepigments or coarse crude pigments, which do not meet industrial requirements. Finely divided prepigments frequently have to be subjected to a thermal treatment to obtain the required properties. Coarsely divided crude pigments are frequently subjected to comminution, which usually has to be followed by thermal treatment in order that pigments meeting the industrial requirements may be obtained.
The thermal treatment may produce various desired effects, for example narrow the particle size distribution; shift the average particle size to higher values; deagglomerate as-ground comminuted and strongly agglomerated pigments; add auxiliaries and distribute them homogeneously over the pigment surface; and effect a change between various crystal forms in some cases. These effects then lead to the desired industrial requirements, for example the desired hue, increased color strength or cleanness of hue, improved dispersibility, rheology, lightfastness, gloss or weatherfastness, or control over light scattering properties and hence over the hiding power.
Literature discloses thermal treatment processes for a wide variety of organic pigments:
DE-A-12 61 106 discloses a batch process for thermal treatment of substituted quinacridone pigments by heating the crude pigments in solvents under pressure.
EP-A-0 318 022 discloses a batch process for producing a hiding dimethyl-perylimide pigment where the hiding power is provided by thermal treatment.
EP-A-0 672 729 discloses a batch process for producing a hiding diketopyrrolo-pyrrole pigment where the hiding power is provided by thermal treatment.
EP-A-0 655 485 and EP-A-0 799 863 disclose batch processes for producing quinacridone pigments where the transformation from the alpha-phase to the beta-phase is effected by thermal treatment with an organic solvent and in the presence of aqueous alkali.
Batch processes are known for producing an azo pigment where a thermal treatment is carried out in an aqueous medium (EP-A-0 077 025) or in organic solvents
(EP-A-0 894 831).
A feature common to these batch processes is the need to control the process parameters. For example, temperature and duration of the thermal treatment, suspension concentration, use of solvents or presence of acids or bases are decisive for the color properties of the pigments obtained and their quality constancy. Moreover, the scaleup of new products from the laboratory scale to the large industrial scale is inconvenient with batch processes and can present problems, since for example vessel and stirrer geometries or heat transfers have a substantial effect on particle size, particle size distribution and color properties.
It is an object of the present invention to provide an environmentally friendly, economical and technically reliable process for preparing organic pigments by thermal treatment that provides very constant adherence to the desired process parameters and simplifies scaleup.
It is known to carry out certain chemical reactions in microreactors. Micro-reactors are constructed from stacks of grooved plates and are described in DE 39 26 466 C2 and U.S. Pat. No. 5,534,328. It is pointed out in U.S. Pat. No. 5,811,062 that microchannel reactors are preferably used for reactions that do not require or produce materials or solids that would clog the microchannels.
It has now been found, that, surprisingly, microreactors are useful for conditioning organic pigments by thermal treatment of their prepigment suspensions.
As used herein, the term xe2x80x9cmicroreactorxe2x80x9d is representative of miniaturized, preferably continuous, reactors which are known under the terms of microreactor, minireactor, micromixer or minimixer and which differ by reason of the dimensions and construction of the channel structures. It is possible to use, for example, microreactors as known from the cited references or from publications of the Institut fucr Mikrotechnik Mainz GmbH, Germany, or of the Fraunhofer Institut fxc3xcr Chemische Technologie, Pfinztal, or else commercially available microreactors, for example Selecto(trademark) (based on Cytos(trademark)) from Cellular Process Chemistry GmbH, Frankfurt/Main.
The invention accordingly provides a process for conditioning organic pigments, which comprises thermally treating a liquid prepigment suspension in a microreactor.
Advantageously, the prepigment suspension is fed to the microreactor continuously. The conventional sequence of adding prepigment suspension, water, organic solvents, acids and/or bases can be realized; similarly, the auxiliaries used in conventional processes may likewise be used in the process of the invention.
Useful organic pigments include, for example perylene, perinone, quinacridone, quinacridonequinone, anthraquinone, anthanthrone, benzimidazolone, disazo condensation, azo, indanthrone, phthalocyanine, triarylcarbonium, dioxazine, aminoanthraquinone, diketopyrrolopyrrole, thioindigo, thiazineindigo, isoindoline, isoindolinone, pyranthrone or isoviolanthrone pigments or mixtures thereof.